Heyel, S. M. and F. P. Day. (Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529). Long-term residual effects of nitrogen addition on a barrier island dune ecosystem. J. Torrey Bot. Soc. 133(2): 297–303. 2006.—In 1991, large plots established on the dunes on Hog Island, part of the Virginia Coast Reserve Long Term Ecological Research site, were fertilized with nitrogen to examine plant community response to nitrogen addition. Nitrogen fertilization significantly increased biomass, and the fertilized plots continued to exhibit enhanced production for 9 years following fertilization.The current study sampled aboveground biomass, belowground biomass, and nitrogen standing crops of the experimental plots on a mid-island dune to determine the mechanism of long- term nitrogen retention in this nitrogen-limited system. Aboveground and belowground biomass were significantly greater in the fertilized plots than in the control plots in 2000. Aboveground biomass declined significantly in control and fertilized plots from 1991 to 2000, while belowground biomass exhibited a significant increase in both control and fertilized plots. In particular, biomass of standing dead, litter, and live roots, and nitrogen standing crop in all plant components were substantially greater in fertilized plots in 2000. Nitrogen concentrations were higher in the litter in the fertilized plots. These data suggest that the retention of nitrogen within the fertilized plots has been primarily facilitated by increased biomass, predominantly in roots, and increased pools of plant litter.